Pumps for implantation in a human patient for long or short-term use as ventricular assist devices (VADs) or complete heart replacement have been explored for some time. It is also well known that pumps for corrosive fluids, blood, and fluids used in food processing may require careful design of the flow passages to avoid fluid damage, contamination, and other undesirable conditions. Accordingly, VADs with magnetically suspended rotors have been developed. Relative to blood pumps, for example, U.S. Pat. Nos. 6,074,180 and 6,394,769, each assigned to the assignee of the present invention and the disclosure of each of which is incorporated, in its entirety, by reference herein, disclose VADs.
With respect to the operation of blood pumps, it is desirable to know the flow rate of blood passing through the blood pump. For example, knowledge of the flow rate through a blood pump may be useful for diagnosing the status of one or more of the following: the performance of the natural heart and related cardiovascular system including things such as, pulse rate, ejection fraction, contractility, opening or closing of the aortic valve or other valves of the heart, operation of valves included in an VAD or similar pump system (e.g., LVAD, RVAD, TAH, BiVAD, etc.), blood flow provided by the natural heart, contractility of the natural heart, hemodynamic condition of the patient, blood flow through a VAD, or properties of the blood.
In order to estimate the flow of blood through a VAD or other blood pump, it is desirable to know the viscosity of the blood. This may allow for a more accurate estimation of the flow through a blood pump. In implantable blood pumps such as left ventricular assist devices (LVADs), conventional approaches have incorporated separate blood flow meters that are implanted with the LVAD. In other conventional systems, no blood flow meter is employed, but other external means are used to make determinations of blood flow, such as, for instance, pump performance, or physiologic state of the patient.
Thus, as mentioned above, one parameter that may be of interest in controlling a pump device having a magnetically suspended rotor is the viscosity of the fluid passing therethrough. Viscosity is a measure of a resistance of a fluid to deformation under shear stress. The viscosity of a liquid is generally related to interaction between constituents comprising the liquid. Generally, a viscosity of a liquid is relatively independent of pressure (except at very high pressure) and usually decreases as a temperature of the liquid increases. Viscosities of liquids are typically several orders of magnitude higher than viscosities of gases.
However, some fluids, such as blood, may include various suspensions, liquids, solids, or cells. In further detail, blood is a suspension which may include red blood cells, white blood cells, and platelets in a plasma of gases, salts, proteins, carbohydrates, and lipids. The viscosity of blood generally increases as the percentage of red blood cells in the blood increases because more red blood cells increase internal friction of the blood, which corresponds to a greater viscosity. The ratio of the volume of packed red blood cells to the volume of whole blood is referred to as the hematocrit. A typical hematocrit of about 40 (that is, approximately 40% of the blood volume is red blood cells and the remainder plasma), may generally correspond to a viscosity of whole blood which is about 2.5 to 3 times a viscosity of water. A hematocrit of about 60 or 70, which may often occur in patients with polycythemia, or abnormally high red blood cell counts, a blood viscosity may become as high as 10 times that of water. Alternatively, when the hematocrit falls drastically, as may occur in patients with anemia (i.e., indicating decreased number of red cells in the blood), blood viscosity can approach that of plasma alone. Further, although the concentrations and types of proteins in plasma can influence the viscosity thereof, such concentrations usually have relatively limited, if any, effect on the overall viscosity of whole blood. Of course, drugs may influence a viscosity of blood.
In an effort to eliminate the need for a conventional, separate flow meter, algorithms that estimate the flow of blood through the blood pump have been created, for example, such algorithms are described in U.S. patent application Ser. No. 10/225,906. These algorithms generally utilize information obtained from the electrical signals used to power the motor that rotates the pump rotor to estimate a flow rate of blood passing therethrough. In the case of magnetically suspended pumps, the electrical signals used to suspend the rotor may also be used to estimate a flow rate of blood passing therethrough. Rotor position sensors may also be employed for such estimations, in addition to other measurements. However, such blood flow estimating algorithms are generally significantly influenced by the viscosity of the blood flowing through the pump. Without a method to determine the viscosity of the blood, the accuracy of these methods is significantly compromised.
Thus, methods for determining or measuring viscosity of fluid passing through a pump device may be of interest. For example, knowledge of a viscosity of a fluid may improve the operation of a pump device. Accordingly, it would be advantageous to provide a system for predicting, estimating, or determining a viscosity of blood that does not require sampling of the fluid passing therethrough.